Variations of CO2 exchange in the Kuye River basin and its influencing factors

Abstract

近年来内陆水体CO2释放受到广泛关注,为揭示黄土高原地区内陆水体CO2的释放特征,于2018年7月和10月及2019年3月和6月利用LI-7000 CO2分析仪对窟野河及代表性水库开展了高频次的水体CO2分压（pCO2）和水-气界面CO2交换通量（FCO2）观测,并分析其时空变化规律。结果表明:窟野河水体pCO2和FCO2(分别为996μatm和94.5 mmol·m-2·d-1)均高于水库(分别为752μatm和10.3 mmol·m-2·d-1)。FCO2季节性差异明显:对于河流而言,表现为秋季最高(165.7 mmol·m-2·d-1),春季最低(42.9 mmol·m-2·d-1);对于水库而言,变化趋势则完全相反,表现为春季最高(16.6 mmol·m-2·d-1),秋季最低(-5.4 mmol·m-2·d-1)。生物地球化学活性更强的支流FCO2(107.4mmol·m-2·d-1)高出干流(66.5 mmol·m-2·d-1)约50%;同时,位于中下游黄土丘陵区的水库FCO2(16.4 mmol·m-2·d-1)显著高于位于上游呼鄂丘陵区的水库FCO2(1.2 mmol·m-2·d-1)。整体来看,流域水体pCO2受碳酸盐体系影响最大,有机碳分解作用次之;流速是控制水-气界面气体交换速率的关键因素。在年尺度上,窟野河的河流与水库水体均为大气持续碳源。窟野河平均CO2释放量与我国长江及国外温带河流相近,但低于黄河中游的其他支流。 This study aimed to examine the riverine CO2 emissions on the Loess Plateau. The river water CO2 partial pressure (pCO2) and CO2 outgassing across the water-air interface (FCO2) in the Kuye River basin, situated in the northern Loess Plateau, was holistically investigated in July and October 2018 and March and June 2019 using a LI-7000 CO2 analyzer. Both pCO2 and FCO2 were higher in rivers (996 μatm and 94.5 mmol·m−2·d−1, respectively) than in reservoirs (752 μatm and 10.3 mmol·m−2·d−1, respectively). Meanwhile, the FCO2 exhibited pronounced seasonal variations. For the river waters, the highest FCO2 of 165.7 mmol · m−2 · d−1 occurred in autumn, and the lowest FCO2 of 42.9 mmol·m−2 ·d−1 occurred in spring. For the reservoir waters, the opposite was observed with the highest FCO2 of 16.6 mmol·m−2·d−1 occurring in spring and the lowest FCO2 of −5.4 mmol·m−2·d−1 occurring in autumn. Spatially, the FCO2 in the tributary rivers (107.4 mmol · m−2 · d−1) with a stronger biogeochemical activity was significantly higher than that in the Kuye mainstream (66.5 mmol·m−2·d−1) by 50%. While for reservoirs, the FCO2 of the reservoir waters (1.2 mmol·m−2·d−1) in the upper sandy hilly area was lower than that in the middle and lower loess hilly area (16.4 mmol·m−2·d−1). In summary, the pCO2 was mostly affected by the carbonate system, followed by dissolved organic carbon. Additionally, flow velocity had a substantial impact on the gas transfer velocity (k), whereas there was no significant correlation between k and wind speed. On an annual scale, both rivers and reservoirs were strong carbon sources for the atmosphere, and their average effluxes were close to that of the Yangtze River while substantially lower than that of the other tributaries in the middle Yellow River Basin.